1. Field of the Invention
The present invention is broadly concerned with new polymers, compositions comprising these polymers, and methods of using these compositions to form a color filter for use in a liquid crystal display. More particularly, the polymers comprise recurring monomers of styrene and maleic anhydride with amino functional groups (e.g., 4-aminoacetophenone, 2-aminophenol, 4-aminobenzoic acid) bonded to at least some of the maleic anhydride monomers.
2. Description of the Prior Art
Color filters are commonly used in liquid crystal display panels, imaging devices, and similar items, and are generally comprise a mosaic of pixels or as a striped pattern. A liquid crystal display is usually produced by forming a multicolor image layer on a transparent glass substrate. The multicolor image layer typically contains red, green, and blue picture elements and, in some cases, also includes a black-colored or black-shaded matrix. A protective layer is then formed on the multicolored image layer, and a transparent conductive layer is subsequently formed on the protective layer, usually by sputtering of indium-tin-oxide (ITO). In the case of a super twisted nematic (STN)-type or a ferroelectric liquid crystal-type panel, the transparent conductive layer is further processed through photolithography to yield a color filter equipped with a transparent electrode, after which an oriented film is formed on the transparent electrode. That substrate and another similarly-formed substrate are sealed together with a gap therebetween, with liquid crystals being poured between the two substrates to yield the display panel.
In the past, the protective layer has been formed on the multicolor image layer for a variety of reasons. For example, the protective layer protects the multicolor image layer from various chemicals (e.g., solvents for the photoresist, acidic etching solutions, basic resist release solutions, solvents for the oriented film coating) used in the photolithographic process carried out to produce the transparent conductive layer. Furthermore, the protective layer gives flatness to the multicolor image layer and assists in maintaining the gap between the two substrates into which the liquid crystals are poured. It is important that the protective layer is transparent (without cloudiness or turbidness) in the visible light region, and that the layer is strongly adhered to both the multicolor image layer and to the substrate.
Recently, there has been a demand to remove the protective layer in order to insure sealing in the non-image portions. As a result, the color layer must be formulated to function in the same manner as the protective layer. Photosensitive resin-type binders are generally used as binders for the color layer in order to enable the layer to meet the above-described requirements. The most popular of these binder resins are those which can be crosslinked by a photo-induced reaction when exposed to UV light. The unexposed portions can then be developed in aqueous alkali solutions which are safe and are the environmentally preferred solutions. The resin is thereafter further cured by heating to give increased chemical resistance.
Color filters containing such a photosensitive resin are prepared by dispersing pigments in the resin. The resin/pigment system also includes dispersing agents which stabilize the pigment dispersion, surfactants which improve the coating smoothness, an addition-polymerizable monomer having an ethylenically unsaturated double bond which improves photo-crosslinking density, and photoinitators for catalyzing the photo-crosslinking reaction.
The color filters must have several other properties in addition to the properties discussed above with respect to the protective layer. For example, the color filter must be able to define a clear pattern of pixels or stripes at small dimensions (e.g., pixel sizes as small as 5 xcexcm). To produce a distinct image without blurriness or fuzziness, the pixels must also be of uniform size and thickness and have straight, clean edges without roughness or pigment residue. Furthermore, the pigments in the composition used in forming the color filter must remain dispersed in the composition over long periods of time, and the composition must also maintain color purity even after prolonged exposure to light. That is, there should be little or no yellowing or discoloration of either the photosensitive resin or the pigments of the composition. Finally, the resulting color filter must be readily processible by standard photolithographic techniques (e.g., the color filter should not require a broadband UV exposure of over 600 millijoules).
Examples of known photosensitive resins for color filters are described in U.S. Pat. No. 5,445,919 to Wakata et al., U.S. Pat. No. 5,514,502 to Wakata et al., and U.S. Pat. No. 5,650,263 to Wakata et al. These patents describe photopolymerizable compositions which comprise a photopolymerization initiator or a photopolymerization initiator system, an addition-polymerizable monomer having at least one ethylenically unsaturated double bond, and a resin obtained by reaction of an anhydride-containing copolymer having a number average molecular weight of 500 to 30,000. This resin has primary amines in a ratio to the copolymer at 0.1 to 1.0 equivalent per equivalent of the anhydride in the copolymer.
Another resin of this type is disclosed in U.S. Pat. No. 5,641,594 to Kudo et al. This composition includes: a polymer having alcoholic or phenolic hydroxyl groups; a compound capable of producing a nitrene when irradiated with an actinic radiation; a pigment; and a solvent. The composition can further contain a heat crosslinking agent and/or a compound having a polymerizable double bond in addition to the above ingredients.
The compositions described in the ""919, ""502, ""263, and ""594 are inadequate for developing color filters with sufficient resolution and small pixel size required in modern high-resolution devices. While these compositions may satisfy some of the requirements described above, none of these resins can meet all color filter needs, especially at 2-4 xcexcm resolution. Furthermore, these compositions require the use of a thermal-crosslinking agent such as tris(acryloloxyethyl) isocyanurate, diisocyanates, or methacrylate melamine compounds. While such additives can improve film hardness and chemical resistance, they are difficult to use and environmentally unfriendly. Isocanates in particular are known carcinogens.
U.S. Pat. No. 5,998,091 to Suzuki describes the synthesis of various monomers, that may be useful for the synthesis of a photosensitive resin which can be used to manufacture color filter materials. While this patent discloses various monomer structures which aid in adhesion to the substrate, alkaline solubility, and pigment dispersion stability, it does not disclose a final polymer composition which can meet the needs described above. In particular, this patent does not disclose a composition which has high photosensitivity, film hardness, and chemical resistance required for color filter materials.
European Patent Application No. 780,731 describes a photosensitive resin which incorporates a phosphoric methacrylate resin, in addition to the photosensitive resin to improve solubility in alkali developer. However, this system discloses nonhomogenous intermixing of the two binder resins which can result in uneven develop characteristics across the entire color filter panel. This is especially true at 2-4 xcexcm pixel sizes, where uniform development is very critical.
U.S. Pat. No. 6,007,966 to Lin describes an alkali-developable, negative acting photosensitive composition which comprises a polymer binder, a photoinitiator, and an unsaturated photomonomer. The polymer binder is prepared from a reaction among a styrene-maleic anhydride resin containing repeating units of styrene and anhydride groups, an unsaturated compound containing at least one hydroxy group and at least three acrylic groups, and a saturated alcohol. This resin system is not optimized for high resolution color filter applications, instead it is designed for the manufacture of printed circuit boards, which have large feature sizes. Thus, this system cannot be used to produce a color filter pattern with clear 2-4 xcexcm resolution.
U.S. Pat. No. 6,051,367 to Kunita et al. a photopolymerizable composition which comprises a compound having one or more addition-polymerizable ethylenically unsaturated bonds and a specific oxime ether compound. However, this compound produces a film where unexposed areas are not developable in aqueous alkali solutions, but instead require a solvent developer. Furthermore, this composition is not suitable for achieving 2-4 xcexcm high resolution patterns.
U.S. Pat. No. 6,027,856 to Nozaki et al. describes a negative-type resist composition which is developable in a basic aqueous solution and comprises a film-formable, basic aqueous solution-soluble polymer with an alkali-soluble group. The composition also includes a compound with an allyl alcohol structure and a photoacid generator which when decomposed by absorption of image-forming radiation causes the compound with an allyl alcohol structure to become a protecting group for the alkali-soluble group, as well as a resist pattern-forming process which employs it. A basic aqueous solution can be used as the developing solution, however, the photoacid generator used in this composition requires an extra hotplate bake process step over conventional color filter materials. Also, as this composition is a negative photoresist, it is not formulated to support pigment dispersions which are used in color filter materials.
The present invention overcomes the problems of the prior art by broadly providing new polymers, compositions comprising these polymers, and methods of using these compositions to form color filters for use in a liquid crystal display.
In more detail, the polymers according to the invention comprise various functional groups (e.g., adhesion promoting groups, photoinitiating groups, solubilizing groups) grafted onto or bonded to at least some of the maleic anhydride monomers of a precursor polymer comprising styrene and maleic anhydride monomers. The precursor polymers can include other monomers in addition to styrene and maleic anhydride, but it is preferred that a styrene-maleic anhydride (SMA) copolymer be utilized. Preferably, the molar ratio of styrene:maleic anhydride in the precursor polymer is from about 1:1 to about 4:1, preferably from about 2:1 to about 4:1, and more preferably about 3:1. The preferred functional groups comprise an aryl moiety with at least one amino group bonded thereto.
The inventive polymer is formed by reacting the selected functional group(s) (either one at a time or simultaneously) with the precursor polymer under conditions to cause at least some of the maleic anhydride rings (preferably at least about 60% of the rings) of the precursor polymer to break and nitrogen atoms from at least one of the respective amino groups to bond to the carbon atom of one of the carbonyl groups on the resulting maleic anhydride moiety. Thus, polymers formed according to the invention will comprise recurring monomers of the formulas 
wherein R1 is a compound selected from the group consisting of xe2x80x94OH and compounds represented by the following formulas: 
xe2x80x83wherein in the above structural formulas:
each R2 is individually selected from the group consisting of hydrogen, xe2x80x94NH2, and xe2x80x94NH;
x is a number ranging from about 1-5, and preferably from about 1-2;
at least one R2 is xe2x80x94NH and the at least one xe2x80x94NH is bonded to one of the carbon atoms labeled with a (1) or a (2);
each R3 is individually selected from the group consisting of hydrogen, xe2x80x94NH2, and xe2x80x94NH;
y is a number ranging from about 0-5, and preferably from about 1-2; and
at least one R3 is xe2x80x94NH and the at least one xe2x80x94NH is bonded to one of the carbon atoms labeled with a (1) or a (2),
with at least one R1 being one of the compounds represented by the above formulas (i.e., a compound other than an xe2x80x94OH group).
In a particularly preferred embodiment, the polymer comprises recurring monomers of at least one of the structures of each class represented in the above formulas (i.e., at least one adhesion promoter, at least one photoinitiator, and at least one solubilizer). Even more preferably, the polymers include recurring monomers of each of the following formulas (i.e., the polymers include 4-aminoacetophenone, 2-aminophenol, and 4-aminobenzoic acid each individually reacted with at least one of the maleic anhydride monomers of the particular polymer). 
In an alternate embodiment, the polymer comprises a photoinitiator bonded thereto. The molar ratio of photoinitiator:maleic anhydride should be from about 1:20 to about 1:1.45, preferably from about 1:20 to about 1:2.5, and more preferably from about 1:10 to about 1:5. The photoinitiator is useful because it improves the degree of crosslinking of color filter compositions including the inventive polymers. Enhanced crosslinking improves pattern resolution, substrate adhesion, film hardness, and chemical resistance of films or layers formed from such filter compositions. A particularly preferred monomer comprising a maleic anhydride moiety reacted with a photoinitiator is represented by the formula 
The polymer preferably further comprises an adhesion promoting group bonded thereto. This is beneficial for forming a pigmented color filter composition which will strongly adhere to the substrate of the liquid crystal display panel thus improving pattern resolution and edge sharpness. The adhesion promoting group should comprise from about 2-50% by weight, preferably from about 2-25% by weight, and more preferably from about 4-10% by weight of the polymer, with the percentage by weight being based upon the total weight of the polymer taken as 100% by weight and referring only to the weight attributable to the adhesion promoting portion of the polymer (i.e., not including the weight of the maleic anhydride moiety to which the adhesion promotor is bonded). The preferred monomer comprising a maleic anhydride moiety reacted with an adhesion promoter is represented by formula 
It is also preferred that the polymer comprise a functional group bonded thereto which acts to improve the solubility of compositions including the inventive polymer in aqueous alkali developers such as tetramethylammonium hydroxide, potassium hydroxide, and sodium carbonate developers. Improved solubility in the developer will lead to improved pattern resolution and edge sharpness of the color filter. This solubility-improving monomer preferably comprises from about 2-50% by weight, preferably from about 2-20% by weight, and more preferably from about 4-10% by weight, with the percentages by weight being based upon the total weight of the polymer taken as 100% by weight and referring only to the weight attributable to the solubilizing portion of the polymer (i.e., not including the weight of the maleic anhydride moiety to which the solubilizer is bonded). A particularly preferred monomer comprising a maleic anhydride moiety reacted with a solubilizing compound is represented by the formula 
The polymers formed according to the invention have an average molecular weight of from about 7,000-13,000 Daltons, and preferably from about 9,000-11,000 Daltons. It will be appreciated that these polymers can be incorporated into compositions which are used to form the pixels on an image layer of a liquid crystal display. Such compositions can be formed according to known preparation procedures. For example, a pigment slurry can be prepared by mixing the desired types and concentrations of pigments with dispersants and/or solvents for the particular pigments. A colored pigment mother liquor can be prepared by mixing a binder resin solution comprising the inventive polymer with the pigment slurry. The resulting mixture is then ground, and a surfactant solution is mixed therewith. The pigment mother liquor is mixed with the desired curing agents, solvents, surfactants, and any other desired ingredients followed by filtering of the mixture to an average particle size of less than about 0.2 xcexcm.
The resulting colored, photosensitive coating composition can then be applied to a substrate (e.g., glass, silicon, ITO glass, color filter layers, etc.) by conventional processes (such as by spin-coating at about 1000 rpm followed by baking on a hotplate at about 100xc2x0 C.). The coated film is exposed through a mask to light at the desired wavelength (e.g., from about 200-500 nm) followed by developing of the film, preferably in a basic developer (e.g., MF312-D27, available from Shipley Company). Finally, the developed film is cured on a hotplate under vacuum at a temperature of from about 150-210xc2x0 C., and the process is repeated as necessary with different pigment colors to obtain differently colored pixels as desired for the particular application (e.g., red, green, and blue pixels), and a protective layer can be applied to the final image layer and cured.
Advantageously, coating compositions utilizing the polymers of the invention give a solvent resistance test result (as defined in Example 7) of less than about 5, preferably less than about 3, and more preferably less than about 2 when PGMEA is used as the solvent. Thus, the resulting films are highly solvent resistant and do not require the use of additional thermal crosslinking agents such as diioscyanates which are hazardous to the user as well as to the environment.
Also, a color filter formed according to the invention comprising an image layer having a thickness of about 1.5 xcexcm will transmit from about 70-95%, preferably from about 80-95%, and more preferably from about 85-90% of light at a wavelength of from about 400-700 nm. Additionally, the inventive color filters will have a resolution of less than about 5 xcexcm, preferably from about 2-4 xcexcm, and more preferably from about 1-2 xcexcm, as determined by a scanning electron microscope. A film formed from compositions comprising the inventive polymers and deposited and cured on a substrate will have a pencil hardness (as determined by JIS K5400) of at least about 4H, and preferably at least about 8H.
Furthermore, films formed from the inventive compositions are highly soluble in alkali developers and exhibit essentially no discoloration or yellowing when exposed to UV light for up to about 2 million lux-hours or when exposed to curing temperatures for time periods of up to about 7 hours. Finally, films formed with the inventive compositions adhere strongly to substrate surfaces and exhibit excellent flatness even over topographical surfaces.